Cancer is the leading cause of death. Currently conducted therapy comprises mainly surgical therapy in combination with radiation therapy and chemotherapy. In spite of the development of new operative procedures and the discovery of new anticancer agents in recent years, cancer treatment results have not been much improved recently, excluding that for some types of cancer. Recent advances in molecular biology or cancer immunology lead to identification of antibodies specifically reacting with cancer, cancer antigens to be recognized by cytotoxic T cells, genes encoding cancer antigens, and the like. Demands on specific cancer therapies targeting cancer antigens are increasing (Non-patent Literature 1).
In cancer therapy, it is desirable that peptides, polypeptides, or proteins recognized as antigens be almost absent in normal cells, but they be present specifically in cancer cells, in order to alleviate side effects. In 1991, Boon et al., (Ludwig Institute for Cancer Research, Belgium) isolated a human melanoma antigen MAGE1 recognized by CD8-positive T cells by the cDNA expression cloning method using autologous cancer cell lines and cancer-reactive T cells (Non-patent Literature 2). Thereafter, the SEREX (serological identification of antigens by recombinant expression cloning) method that comprises identifying tumor antigens recognized by antibodies that are produced in vivo in response to autologous cancer of a cancer patient by gene expression cloning techniques was reported (Non-patent Literature 3 and Patent Literature 1). With the use of this method, some cancer antigens, which are almost not expressed in normal cells but are expressed specifically in cancer cells, were isolated (Non-patent Literatures 4-9). Furthermore, clinical trials were conducted with cell therapies targeting some cancer antigens using immunocytes specifically reactive with cancer antigens, or cancer-specific immunotherapies using vaccines or the like containing cancer antigens.
Meanwhile, in recent years, various antibody medicines which target antigenic proteins on cancer cells for cancer treatment have appeared throughout the world. Antibody medicines exhibit some pharmacological effects as cancer specific therapeutic agents and are thus attracting attention. However, most antigen proteins to be targeted are also expressed in normal cells, so that not only cancer cells, but also normal cells expressing antigens are also damaged as a result of antibody administration. The resulting side effects are a cause for concern. Therefore, it is expected that identification of cancer antigens that are specifically expressed on the surface of a cancer cell and use of antibodies targeting the cancer antigens as pharmaceuticals will realize treatment with antibody medicines with lower side effects.
Cytoplasmic- and proliferation-associated protein 1 (CAPRIN-1) is expressed when normal cells at the resting phase are activated or undergo cell division, and it is an intracellular protein known to form intracellular stress granules with RNA within cells, so as to be involved in mRNA transport and translational regulation. Meanwhile, many other names that represent CAPRIN-1 exist, such as GPI-anchored membrane protein 1 or membrane component surface marker 1 protein (M11S1), as if such proteins had been known to be cell membrane proteins. These names originated from a report that the gene sequence of CAPRIN-1 is a membrane protein having a GPI-binding region and expressed in colorectal cancer cells (Non-patent Literature 10). However, the gene sequence of CAPRIN-1 provided in this report was later revealed to be wrong. The following has recently been reported; i.e., deletion of a single nucleotide in the gene sequence of CAPRIN-1 registered at GenBank or the like causes a frame shift, so that 80 amino acids are lost from the C-terminus, resulting in generation of an artifact (74 amino acids) which corresponds to the GPI-binding portion in the previous report, and additionally, another error is also present 5′ of the gene sequence, so that 53 amino acids were lost from the N-terminus (Non-patent Literature 11). It has been also recently reported that the protein encoded by the gene sequence of CAPRIN-1 registered at GenBank or the like is not a cell membrane protein (Non-patent Literature 11).
In addition, on the basis of the report of Non-patent Literature 10 that CAPRIN-1 is a cell membrane protein, Patent Literatures 2 and 3 describe that CAPRIN-1 (as a cell membrane protein) under the name of M11S1 can be used as a target of an antibody medicine in cancer therapy, although working examples do not describe treatment using an antibody against the protein. However, as reported in Non-patent Literature 11, it has been commonly believed from the time of the filing of Patent Literature 2 to date that CAPRIN-1 is not expressed on the surface of a cell. The contents of Patent Literatures 2 and 3 based only on incorrect information that CAPRIN-1 is a cell membrane protein should not clearly be understood as common general knowledge for persons skilled in the art.